Recent assessment of material property requirements for blast resistant applications, especially for the naval ship hulls, has defined the need to design steels with high stretch ductility and fragment penetration resistance, along with high strength and adequate toughness. Advancement in new systems-based design methodology which accelerates the total product development life-cycle while achieving new levels of product reliability has led to rapid innovations in steel technology and design. In an effort to create materials with maximum durability for varied applications ranging from hull steels for Naval warships, aircraft landing gears to high performance engines, most of recent steel research initiatives have mainly focused on achieving extremely high strength or toughness or combinations of both accompanied with good weldability and corrosion resistance. However, in the wake of current needs of the Navy in specific where high blast impulse resistance coupled with fragmentation/shear resistance is desired for any new alloy design, it has been recognized that an ideal performance criterion in addition to high strength would be to have high uniform ductility under both tension and shear loads.
The particular challenge of the current design problem is to achieve both strength and fracture toughness while maintaining high uniform ductility and shear resistance at room temperature; usually with the gain of one comes the loss of the other. The use of austenitic Transformation-Induced Plasticity (TRIP) steels designed earlier [reference 1] allows plastic flow stabilization that can be applied to either uniform ductility or toughness. This austenite to martensite transformation is influenced by temperature, applied stress, composition of the alloy, strain-rate, stress-state and any prior deformation of parent austenite [reference 2]. The mechanism of the transformation and the kinetics governing it have been well established by Olson and Cohen [references 3-5] and have been used to generate constitutive equations and models to determine the stability of the parent matrix phase, which is critical in determining ideal transformation temperature and other parameters. The transformation to martensite provides resistance to necking in tension thereby increasing not only the uniform ductility but also the ultimate tensile strength (UTS) [reference 6]. This transformation behavior is dependent on the stability of the austenite matrix and its influence on the mechanical properties of TRIP steels have been extensively studied by Bhandarkar et al. [reference 7].
While the martensitic BlastAlloy160 [reference 8] steel was designed based on the initial assumption that toughness is the critical factor in blast protection, recent computer simulations and failure analysis [reference 9] have indicated that uniform ductility is the limiting property for impulse resistance, provided a critical toughness is maintained to avoid shattering. This reassessment of requirements has changed the goals of the current design to develop a prototype which has improved ductility at high yield stress levels with just sufficient fracture toughness. These objectives should be met while maintaining the other properties desirable for naval hull steels, such as non-ferromagnetism (for reduced magnetic signature at use temperatures), good weldability, and resistance to hydrogen-stress-corrosion cracking. Based on these requirements, the following property objectives have been defined:    1. To achieve Yield Strength of ˜120 ksi and UTS >130 ksi.    2. To achieve at least 20% uniform elongation under tension and shear loading conditions at room temperature, with significant necking (>50% in Reduction of Area)    3. To maintain a high fracture toughness (greater than 90 ksi/in0.5)—since a goal of this invention is improved ductility, the fracture toughness requirements have been lowered.    4. To be non-magnetic at use temperatures—the invention will constrain the Curie temperature, Tc of the alloy composition below room temperature.    5. To be easily weldable    6. To be resistant to environmental hydrogen and stress-corrosion cracking    7. Low Cost
Most of the commercially available steels used to build hulls of ships, such as the A286 and HSLA 100 steels, have an inadequate combination of strength-toughness—ductility properties. An increase in one of these properties leads to the decrease in the other and combined with other material characteristics, such as weldability and low cost, these alloy steels do not serve the necessary objective of adequate resistance against blast impulse explosions and fragments.